Co-writing can be an effective and enjoyable way of writing. It must be done right: see Simon Whaley’s feature Writing Tandem in the May issue.
Having co-written several (non-fiction) books I can confirm that this provides a practical review of the way to go about it, and that it does need a systematic approach as well as compatible creativity to make it work. One of those quoted, Peter Jones (who wrote How to Eat Loads and Stay Slim with Della Galton), said ‘It took about half the time to write this book as my other self-help books.’ So as well as being a viable way to write, perhaps by pooling expertise allowing writing that neither party could do alone, it can also positively affect productivity.